Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, Flash, etc. A MRAM device is generally comprised of an array of parallel first conductive lines on a horizontal plane, an array of parallel second conductive lines on a second horizontal plane spaced above and formed in a direction perpendicular to the first conductive lines, and an MTJ element interposed between a first conductive line and a second conductive line at each crossover location. A first conductive line may be a word line while a second conductive line is a bit line or vice versa. Alternatively, a first conductive line may be a bottom electrode that is a sectioned line while a second conductive line is a bit line (or word line). There are typically other devices including transistors and diodes below the array of first conductive lines as well as peripheral circuits used to select certain MRAM cells within the MRAM array for read or write operations.
An MTJ element may be based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. In an MRAM device, the MTJ element is formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line. An MTJ stack of layers that are subsequently patterned to form an MTJ element may be formed in a so-called bottom spin valve configuration by sequentially depositing a seed layer, an anti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer, a thin tunnel barrier layer, a ferromagnetic “free” layer, and a capping layer. The AFM layer holds the magnetic moment of the pinned layer in a fixed direction. In a MRAM MTJ, the free layer is preferably made of NiFe because of its reproducible and reliable switching characteristics as demonstrated by a low switching field (Hc) and switching field uniformity (σHc). Alternatively, an MTJ stack may have a top spin valve configuration in which a free layer is formed on a seed layer followed by sequentially forming a tunnel barrier layer, a pinned layer, AFM layer, and a capping layer.
The pinned layer has a magnetic moment that is fixed in the “y” direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the “y” direction. The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed from the top electrode to the bottom electrode in a direction perpendicular to the MTJ layers, a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state.
In a read operation, the information stored in an MRAM cell is read by sensing the magnetic state (resistance level) of the MTJ element through a sense current flowing top to bottom through the cell in a current perpendicular to plane (CPP) configuration. During a write operation, information is written to the MRAM cell by changing the magnetic state in the free layer to an appropriate one by generating external magnetic fields as a result of applying bit line and word line currents in two crossing conductive lines, either above or below the MTJ element. In certain MRAM architectures, the top electrode or the bottom electrode participates in both read and write operations.
A high performance MTJ element is characterized by a high magnetoresistive (MR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the change in resistance observed by changing the magnetic state of the free layer. A high MR ratio of over 130% and a low magnetostriction (λS) value of about 1×106 or less are desirable for Spin-RAM applications. This result is accomplished by (a) well controlled magnetization and switching of the free layer, (b) well controlled magnetization of a pinned layer that has a large exchange field and high thermal stability and, (c) integrity of the tunnel barrier layer. In order to achieve good barrier properties such as a specific junction resistance×area (RA) value and a high breakdown voltage (Vb), it is necessary to have a uniform tunnel barrier layer which is free of pinholes that is promoted by a smooth and densely packed growth in the AFM and pinned layers. Although a high RA value of about 10000 ohm-μm2 is acceptable for a large area (A), RA should be relatively small (<1000 ohm-μm2) for smaller areas. Otherwise, R would be too high to match the resistance of the transistor which is connected to the MTJ.
Generally, the purpose of the capping layer is to protect underlying layers in the MTJ during etching and other process steps and to function as an electrical contact to an overlying conductive line. The typical capping layer for an MTJ stack is a non-magnetic conductive metal such as Ta or TaN. During thermal annealing, Ta is capable of gettering oxygen atoms originating in the NiFe free layer. Consequently, the NiFe free layer is less oxygen contaminated and a more distinct boundary between the tunnel barrier layer and NiFe free layer is thereby obtained to improve dR/R. The disadvantage of using a Ta capping layer is that Ta diffuses into NiFe during thermal annealing, especially at high annealing temperatures (i.e. >250° C.) to produce an alloy that not only reduces free layer moment (Bs) but makes NiFe very magnetostrictive with a λS of ≧5×10−6. Thus, alternative capping layer materials are desirable that minimize inter-diffusion between a free layer and capping layer, serve as a good oxygen getter material, and enable both a high MR ratio and low λs value to be achieved in MTJs for advanced MRAM and TMR read head technologies.
Spin transfer (spin torque) magnetization switching as described by J. Sloneczewski in “Current-driven excitation of magnetic multilayers”, J. Magn. Materials V 159, L1-L7 (1996), and by L. Berger in “Emission of spin waves by a magnetic multiplayer traversed by a current” in Phys. Rev. Lett. B, Vol. 52, p 9353 has stimulated considerable interest in recent years due to its potential application for spintronic devices such as MRAM on a gigabit scale. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high, and if the dimensions of the multilayer are small. The difference between a Spin-RAM and a conventional MRAM is only in the write operation mechanism. The read mechanism is the same.
Referring to FIG. 1, a memory cell 1 of a Spin-RAM includes a MTJ 13, word line (WL) 6, bit line (BL) 14, bottom electrode 7, and a CMOS transistor having a source 3, drain 4, and p-type semiconductor 2, for example, that provides current for switching the MTJ free layer 11. There is also a gate electrode 5. Additional layers in the MTJ 13 are an AFM layer 8, pinned layer 9, insulating barrier 10, and capping layer 12.
A critical current for spin transfer switching (Ic), which is defined as [(Ic++I Ic− I)/2], for the present 180 nm node sub-micron MTJ having a top-down area of about 0.2×0.4 micron, is generally a few milliamperes. The critical current density (Jc), for example (Ic/A), is on the order of several 107 A/cm2. This high current density, which is required to induce the spin-transfer effect, could destroy a thin insulating barrier 10 such as AlOx, MgO, or the like. In order for spin-transfer magnetization switching to be viable in the 90 nm technology node and beyond, the critical current density (Jc) must be lower than 106 A/cm2 to be driven by a CMOS transistor that can typically deliver 100 μA per 100 nm gate width. For Spin-RAM applications, the (ultra-small) MTJs must exhibit a high tunnel magnetoresistance ratio (TMR or dR/R) much higher than the conventional MRAM-MTJ that use AlOx as a barrier layer and have a dR/R of about 40% as stated by Z. Diao et. al in “Spin transfer switching and spin polarization in MTJ with MgO and AlOx barrier”, Appl. Phys. Left, 87, 232502 (2005). D. Djayaprawira et. al in “230% room temperature magnetoresistance in CoFeB/MgO/CoFeB MTJ”, Appl. Phys. Lett. V 86, p. 092502 (2005) demonstrated that a highly oriented (001) CoFeB/MgO/CoFeB MTJ is capable of delivering dR/R>200%. Therefore, it is essential to find a way to combine a high TMR ratio of a CoFeB/MgO/CoFeB MTJ and the current driven switching capability necessary to make Spin-RAM a practical technology.
To apply spin-transfer switching to MRAM technology, it is desirable to decrease Ic (and its Jc) by more than an order of magnitude so as to avoid an electrical breakdown of the MTJ device and to be compatible with the underlying CMOS transistor that is used to provide switching current and to select a memory cell. A means to improve the dielectric breakdown voltage is also an important consideration.
The intrinsic critical current density (Jc) as given by Slonczewski of IBM is shown in equation (1) below.Jc=2eαMstF(Ha+Hk+2πMs)/ℏη  (1)where e is the electron charge, α is a Gilbert damping constant, tF is the thickness of the free layer, ℏ is the reduced Plank's constant, η is the spin-transfer efficiency which is related to the spin polarization (P), Ha is the external applied field, and Hk and Ms are respectively, uniaxial anisotropy and magnetization of the free layer.
Normally, the demagnetizing field, 2πMs (several thousand Oe term) is much larger than the uniaxial anisotropy field Hk and external applied field (approximately 100 Oe) Ha term, hence the effect of Hk and Ha on Jc are small. In equation (2), V equals MS (tFA) and is the magnetic volume which is related to the thermal stability function term KuV/kbT where Ku is the magnetic anisotropy energy and kb is the Bolzmann constant.Jc∝αMsV/P  (2)
A routine search of the prior art was conducted and the following references were found. Hosomi et al. in “A novel non-volatile memory with spin torque transfer magnetization switching: Spin-RAM”, 2005 IEDM, paper 19-1, present a Spin-RAM with spin-torque transfer magnetization switching for the first time. Sony's Spin-RAM devices were fabricated with a Co40Fe40B20/RF sputtered MgO/Co40Fe40B20 (pinned layer/tunnel barrier/free layer) MTJ configuration that was processed with a 350° C.-10K Oe annealing. The CoFeB/MgO/CoFeB MTJ is employed for its high polarization (P) that provides a high output signal for TMR. MTJ size is 100 nm×150 nm with an oval shape. A tunnel barrier layer is made of crystallized (100) MgO whose thickness is controlled to <10 Angstroms for the proper RA of about 20 ohm-μm2 while dR/R or TMR (intrinsic) of the MTJ is 160%. Using a 10 ns pulse width, the critical current density, Jc, for spin transfer magnetization switching is about 2.5×106 A/cm2 which means Ic is equal to 375 μA. Due to a very small MTJ size, resistance distribution of Rp (low resistance state) and Rap (high resistance state) has a sigma (Rp_cov) around 4%. Thus, for a read operation, TMR (without bias)/Rp_cov=40 and this ratio is equivalent to that for a conventional CoFeB/AlOx/NiFe (pinned layer/tunnel barrier/free layer) MRAM MTJ configuration in which TMR is typically 40% with an Rp_cov of around 1%. Note that MTJ size in this case is 300 nm×600 nm. In addition, a CoFeB/AlOx/NiFe MRAM MTJ in a read operation is typically 300-350 mV biased. Under this condition, TMR (350 mV bias)/Rp_cov would be reduced to about 20.
A spin transfer magnetization switching of a Co60Fe20B20/MgO/Co60Fe20B20 MTJ is reported by Y. Huai et al. in “Spin transfer switching current reduction in magnetic tunnel junction based dual filter structures” in Appl. Physics Lett., V 87, p. 222, 510 (2005). The nominal MTJ size is 125 nm×220 nm with an RA of about 50 ohm-μm2 and dR/R=155%. Jc0 (i.e. Jc extrapolated to a pulse width of 1 ns) is ˜2×106 A/cm2, similar to the Sony example. For a dual spin filter (DSF) structure wherein free layer switching is affected by two spin torques, Jc has been reduced to ˜1.3×106 A/cm2.
In another reference by J. Hayakawa et al. entitled “Current-driven magnetization switching in CoFeB/MgO/CoFeB magnetic tunnel junctions”, Japan J. Appl. Phys. V 44, p. 1267 (2005), a Jc (at a 10 ns pulse width) is reported as 7.8×105, 8.8×105, and 2.5×106 A/cm2 for MTJs processed with 270° C., 300° C., and 350° C. annealing temperatures, respectively. RA for the MTJ that has an 8.5 Angstrom MgO tunnel barrier thickness is about 10 ohm-μm2. TMR (intrinsic) or dR/R ratios as a function of the three annealing temperatures for the Co40Fe40B20/MgO/Co40Fe40B20 MTJs with a 20 Angstrom thick Co40Fe40B20 free layer are 49%, 73%, and 160%, respectively. It was found that a CoFeB free layer which is annealed at 270° C. or 320° C. is amorphous as described by S. Cardoso et. al in “Characterization of CoFeB electrodes for tunnel junction”, J. Appl. Phys., V 97, p. 100916 (2005). On the other hand, a CoFeB free layer annealed at 350° C. is crystalline. It has been confirmed that the damping constant for an amorphous CoFeB layer is about half that of a crystalline CoFeB layer by C. Bilzer et. al in “Study of the dynamic magnetic properties of soft CoFeB films”, J. Appl. Phys., V 100, p. 053903 (2006). Amorphous layers showed a low damping (α=0.006) that is thickness dependent while crystalline CoFe with no B content has a value 2× higher (α=0.013).
The aforementioned references can be summarized with the following four points: (1) Jc is greater than 2×106 A/cm2 for a CoFeB20/MgO/CoFeB20 MTJ having a crystalline free layer; (2) a Jc less than 1.0×106 A/cm2 can be achieved for a MTJ with an amorphous CoFeB free layer although dR/R is less than 100%; (3) a TMR (with bias)/Rp_cov (4%)>20 is required for a CoFeB20/MgO/CoFeB20 MTJ to be useful in a Spin-RAM application; and (4) Jc can be reduced in half by employing a dual spin filter (DSF) MTJ structure. However, to our knowledge, none of the prior art references has achieved a Jc less than 1.0×106 A/cm2 and a dR/R over 120% together with a TMR (300 mV bias)/Rp_cov (4%) of at least 20 which is believed to be necessary for a Spin-RAM application. Furthermore, it is believed that the DSF MTJ structure may be too difficult to manufacture and that it is more desirable to fabricate a Spin-RAM device using a MTJ having a single spin valve structure.
In other prior art references, U.S. Pat. No. 6,831,312 discloses a list of amorphous alloys such as CoFeB and CoFeHf. Crystallization temperature for CoFeHf is >350° C. and for CoFeB is 325° C.-350° C. An amorphous free layer of CoFeB is also described in U.S. Patent Application 2005/0277206 and in U.S. Patent Application 2006/0003185. U.S. Pat. No. 6,990,014 teaches that an information recording layer comprises amorphous CoFeB. U.S. Patent Application 2006/0209590 discloses an amorphous CoFeB layer over a MgO tunnel barrier layer.